Mineral Database
Cumengéite
Crystal System: Tetragonal
Formula: Cu20Pb91Cl47(OH)40
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Lead copper chloride hydroxide
Method(s) of Verification: Abersoch - XRD (Manchester Museum, MANCH:XRD378).
Chemical Group:
- Halides
Geological Context:
- Supergene : post-mining oxidation & weathering deposits
Introduction:
cumengéite is a supergene mineral, the formation of which requires a particularly high concentration of chloride ions to be present. This rather specific requirement means that the environmental conditions for the formation of cumengéite are of a tightly-constrained nature (Humphreys et al., 1980). Its single Welsh occurrence, within an intertidal zone (where the chloride concentration is increased by evaporation of sea-water) and in association with other chloride-bearing phases, is consistent in such terms to the small number of other occurrences reported in Britain.
Occurrence in Wales:
only one very limited occurrence of cumengéite is known from Wales, on a beach on Llŷn adjacent to a number of disused metal mines (Dossett & Green, 1998).
Key Localities:
- Abersoch, Gwynedd: cumengéite occurs but is rare on Abersoch beach, forming light blue crusts which are composed of minute (<0.1 mm) bipyramidal crystals (Dossett & Green, 1998). Associated minerals are atacamite and rare botallackite. The host matrix consists of pebbles of vein-breccia cemented by quartz and containing chalcopyrite and galena.
References:
- Atacamite, botallackite and cumengeite from Abersoch, Gwynedd, Wales. Journal of the Russell Society 7(1), 38-39.
- The chemical stability of mendipite, diaboleite, chloroxiphite and cumengéite, and their relationships to other secondary lead(II) minerals. Mineralogical Magazine, 43, 901-904.
